The present invention relates to a thrust roller bearing, and prevents thrust needle rollers from coming out of a cage.
In the thrust roller bearing of the related art having rollers and a cage for holding the rollers, the cage is generally made of a press molding of a thin steel plate, and has ribs at the radially outer side end portion and at the radially inner side end portion. Moreover, pockets for mounting a plurality of rollers of the cage are so formed by a punching work and have a structure in which bent portions positioned across the individual PCD (pitch circle diameter) positions of the rollers in the axial direction are formed at two radial end portions and at the central portions of the individual pockets. The individual rollers are prevented (as referred to Patent Document 1, for example) from coming out in the axial direction of the cage (in the vertical direction) by the roller retaining portions formed at the bent portions in the pockets of the cage.    Patent Document 1: JP-UM-A-64-55322
In this thrust roller bearing of the related art, in the preassembled state before the assembly with a shaft, the rollers are retained in the pockets of the cage. However, the roller is inclined when an external force acts on the roller. Then, the end surfaces of the roller are brought out of engagement with the roller end surface guiding surfaces of the pocket due to the inclination of the roller, so that the roller moves toward the rib side. At this time, the rib inner surfaces act as the surfaces to prevent the roller from coming out.
In the cage structure of the related art provided with three bent portions at the pocket, on the other hand, the radial length of the pocket becomes short, especially in case the roller used has a small entire length. As a result, the roller retaining portions disposed at each bent portion become narrow and hard to form precisely.
As a result, when the roller is inclined to move so far that its end surface is supported by the inner surface of the rib, the roller is not caught on the roller end surface side on the opposite side by the roller retaining portion but may not be retained thereby the rollers coming out.
Here, in order to reduce the movement of the inclined roller till the roller end surface is supported by the inner surface of the rib of the cage, it is necessary to minimize the clearance between the radial end surface portion of the pocket for guiding the roller end surface and the aforementioned rib inner surface. However, the rib is formed by the pressing, and therefore the reduction in the clearance is limited and dispersed.
In case the clearance is enlarged by that dispersion, therefore, the rib inner surface does not act as the stopping surface, and the roller may come out of the cage.
On the other hand, in order to relax the stress, the cage is requested to make the radius of curvature of the four inner corners of the pocket as large as possible, whereas the roller is requested to make its effective length as large as possible thereby to increase the load capacity of the bearing. For this request, it is desired to enlarge the effective length of the roller by reducing the chamfer of the end surfaces of the roller. In order to avoid the interference of the two end edges of the roller with the four inner corners of the pocket, however, the radius of curvature of the inner corners is set smaller than that of the round chamfers at the end surfaces of the roller. As a result, the cage may have an insufficient fatigue strength.